The present invention is directed to a method for making a golf ball. More particularly, the invention is directed to a method for making a multilayered golf ball having a plurality of core parts or cups arranged around a center and bound to each other by crosslinking or an adhesive.
Generally, golf balls have been classified as two piece balls or three piece balls. Two piece balls are comprised of a solid polymeric core and a cover. These balls are generally easy to manufacture, but are regarded as having limited playing characteristics. Three piece balls are comprised of a solid or liquid-filled center surrounded by tensioned elastomeric material and a cover. Three piece balls generally have a good xe2x80x9cclickxe2x80x9d and xe2x80x9cfeelxe2x80x9d when struck by a golf club, but are more difficult to manufacture than two piece balls.
The prior art is comprised of various golf balls that have been designed to provide optimal playing characteristics. These characteristics are generally the initial velocity and spin of the golf ball, which can be optimized for various players. For instance, certain players prefer to play a ball that has a high spin rate for playability. Other players prefer to play a ball that has a low spin rate to maximize distance. However, these balls tend to be hard feeling and difficult to control around the greens.
The prior art is comprised of liquid filled golf balls. Wound golf balls have been made with liquid filled centers for many years. Both U.S. Pat. Nos. 1,568,513 and 1,904,012 are directed to wound golf balls with liquid filled centers. U.S. Pat. Nos. 5,150,906 and 5,480,155, are directed to a hollow spherical shell of a polymeric material which is filled with a liquid or unitary, non-cellular material that is a liquid when introduced into the shell. The shell is disclosed as being the outer cover or an inner layer with the outer cover formed to the external surface thereof. The shell varies in thickness from about 0.060 to 0.410 inches in thickness.
Other known attempts to mold layers around a solid center entail placing a preformed center between two blocks of core material in a spherical compression mold, and closing the mold. This is done in the manufacture of golf balls sold by Kamatari. This process, however, provides little control over the ultimate placement of the center within the golf ball core. Large variations in center eccentricities can result.
The prior art also provides for the manufacture of double cover golf balls. This is generally accomplished by injection molding a first and then a second cover layer around a core. This system, however, requires complex injection molds, usually with retractable pins within the mold to properly position the core.
The invention provides a method for making a golf ball, particularly suited for golf balls that have a multilayer core with a fluid center, and also a golf ball resulting from the inventive process. The method comprises forming a solid or fluid filled inner sphere, and then separately molding from elastomeric material, preferably polybutadiene, core parts, such as hemispherical cups. When the cups are combined a hollow sphere is formed.
The each hemispherical cup has a hemispherical cavity, produced by a protrusion of a mold part. The inner sphere is placed between two cups, which are then joined to form a liquid center shell of the core. When the cups are joined, the hemispherical cavities together form a spherical cavity, now occupied by the inner sphere, and the cups themselves form a liquid center shell of the core. Thus, the inner sphere is easily positioned concentrically within the finished ball. Finally, a cover is molded around the core. This process results in accurate and repeatable central placement of the inner sphere within the core.
The process is not susceptible to unwanted deformation of a soft inner sphere during placement within the cups"" cavities because the method does not depend on the rigidity of the inner sphere in the shaping of the cups. Moreover, as the cups are molded separately from the inner sphere, the process avoids deforming a soft inner sphere containing a fluid because compressing the cup material about the inner sphere is not required.
In a method according to the invention, the two cups are molded on both sides of a single protrusive mold part that has a hemispherical protrusion on each side for forming each hemispherical cavity.
Once the cups are formed, they are joined by applying an adhesive between them and pressing them together. The adhesive employed also has an adhesive strength that is preferably stronger than the cohesive strength of the elastomeric core material. The adhesive is preferably flexible in its cured state.
An alternative method to employing adhesive is to join the cups by pressing them together while their temperature is elevated to crosslink the material from one cup to the other.
The invention also provides making hemispherical cups with nonplanar mating surfaces. The mating surfaces are substantially flat in one embodiment, but, in another embodiment, define a pattern that is symmetrical about the inner sphere, such as of concentric ridges that mesh when the two cups are joined. These ridges may comprise a corresponding tongue and groove. The nonplanar surfaces improve the shear strength of the finished ball where the cups are joined and retain more characteristics of the cup material throughout the joining region than do balls with flat mating surfaces.
Properties of golf balls according to the invention, such as spin rate, spin decay, compression, and initial velocity, may be varied, for example by selecting appropriate specific gravities and viscosities of a fluid in the inner sphere. The fluid may have a high specific gravity and low viscosity for a high spin ball or a low specific gravity and high viscosity for a low spin ball. The properties of the liquid center shell or one or more mantle layers in the ball, such as specific gravity, resiliency, and compression can also be varied to make balls having the desired characteristics.